1. Field of the Invention
The invention relates to an image forming apparatus of a copying apparatus, a printer, or the like and, more particularly, to an image forming apparatus for clearly reproducing characters and fine lines in the case of forming an image in which an image area of a half tone such as photograph, screen (or dot), or the like mixedly exists oils characters and fine lines.
2. Description of the Related Background Art
In recent years, the number of full-color images being generated has increased, not only in the field of print or design, but also in the field of offices or the like. Consequently, color copying apparatuses which read color original documents at a high fidelity and which generate full-color images have also become wide spread. In those color copying apparatuses, the requirement to generate a full-color image at a high gradation, and the requirement to generate a clear color character original at a high resolution are simultaneously needed.
A dither method, a screen dot modulating method, and the like are known as methods of generating a full-color image at a high gradation. However, when those methods are applied to characters or a line image, the resolution remarkably decreases and the character quality deteriorates. On the other hand, a binary process is suitable to preferably reproduce characters or a line image. However, as is well known, when the binary process is applied to a screen or photograph image, the gradation remarkably decreases and the picture quality deteriorates as well.
Many methods have been proposed to reconcile character quality half-tone quality. For instance, in an image reading apparatus, there have been proposed methods whereby a black area is first extracted by lightness information and saturation information of an original document, an edge portion of the original document is further extracted to thereby discriminate the black edge portion, a black character and a black fine line in a color image or a screen are automatically discriminated on the basis of a degree of black edge and a degree of color information near the black edge portion by referring to the color information near the black edge portion, and an image signal including a signal to designate image forming conditions is generated to an image forming apparatus (U.S. patent application Ser. No. 173,654 filed on Mar. 25, 1988, U.S. Pat. No. 4,958,219, U.S. patent application Ser. No. 327,098 filed on Mar. 22, 1989, U.S. Pat. No. 5,031,034, U.S. patent application Ser. No. 519,500 filed on May 4, 1990 U.S. Pat. No. 5.113.252, and U.S. patent application Ser. No. 267,366 filed on Nov. 4, 1988.
In the above image forming apparatus, images are respectively formed by first and second image forming circuits suitable for a half-tone image reproduction and a line image reproduction in accordance with the image forming conditions.
For instance, a laser beam printer having advantages such that a print quality is high, a print speed is high, and such a printer is wisely used as an output device of a color copying apparatus or the like.
FIGS. 2A and 2B are block constructional diagrams of an image forming apparatus such as a laser beam printer. The conventional image forming apparatus will now be described hereinbelow with reference to the drawings. In the conventional apparatus, a laser beam copying apparatus of the electrophotographic type which scans a laser beam onto a photo sensitive drum synchronously with the reading of an original document to thereby form an image will be explained as a typical example. The above principle has been disclosed in, for instance, U.S. Pat. No. 4,800,442.
An original document 9 is first read by a CCD 1. A resultant analog image signal is amplified to a predetermined level by an amplifier 2 and is converted into a digital image signal of eight bits (gradations 0 to 255) by an A/D converter 3. The digital image signal is transmitted through a gradation control circuit 10 (converting circuit to perform a digital conversion by a measure of a look-up table constructed by an RAM of 256 bytes) and is gradation corrected and is supplied to a D/A converter 14.
The digital signal is again converted into the analog signal by the D/A converter 14. The analog signal is compared with a signal of a predetermined period which is generated from a triangle wave generation circuit 15 by a comparator 16 and is pulse width modulated. The pulse width modulated binary image signal is directly supplied to a laser drive circuit 17 (FIG. 2B) and is used as a signal to on/off control the light emission of a laser diode 18. A laser beam emitted from the laser diode 18 is scanned in the main scanning direction by a well-known polygon mirror 19 and is transmitted through an f/.theta. lens 20 and is reflected by a reflecting mirror 21. The laser beam is subsequently irradiated onto a photo sensitive drum 22 as an image carrier which is rotating in the direction shown by an arrow, thereby forming an electrostatic latent image thereon.
After the photo sensitive drum 22 is uniformly discharged by an exposing device 28, it is uniformly charged to a minus potential by a charging device 23. After that, the laser beam is irradiated onto the drum 22 and an electrostatic latent image corresponding to an image signal is formed on the surface of the drum 22. What is called an image scanning method of exposing a portion (black pixel) to be developed is then executed (this is frequently used in laser beam printers). Therefore, by a well-known inverse developing method, a developing device 24 deposits toner having minus charging characteristics to the portion of the drum 22 which has been discharged by the laser beam and develops the latent image.
The developed image (toner image having minus charges) formed on the drum 22 by the above method is copy transferred onto a copy transfer material (generally, paper) 26 by a copy transfer charging device 25.
The visible toner image, now transferred to the copy transfer material 26, is then fixed by a fixing device (not shown).
The residual toner remaining on the drum 22 is scraped off by a cleaner 27. The above processes are again repeated.
In the image forming apparatus as mentioned above, in order to clearly reproduce a line image such as, particularly, characters and fine lines or the like, the line image and a half-tone image are discriminated in the image reading apparatus (as described above), when the line image and half-tone image are formed, a period of the reference pulse which is used in the pulse width modulation process is changed. More particularly, a plurality of gradation reproducing characteristics as shown at a and b in FIG. 3 are selected and the reproducing characteristic a is selected for the half-tone image, thereby assuring the adequate gradation reproduction in a range from a low density portion to a high density portion, while the gradation reproducing characteristic b is selected for the line image, thereby executing the gradation reproduction of an edge emphasis, so that the characters and line image can be clearly reproduced.
When the line image and half-tone image are formed, by changing a light emission quantity of the laser diode 18, a plurality of gradation reproducing characteristics similar to those mentioned above are selected and the gradation reproduction suitable to form the half-tone image and the line image can be also executed.
In the above conventional example, however, by selecting the gradation reproducing characteristic as shown at b in FIG. 3 to a fine image, although characters and fine line of high densities are more clearly reproduced, there is a problem such that a micro fluctuation of the density is emphasized for characters or fine line having low densities and the sharpness is contrarily lost.
To solve the above problem, accordingly, there is used a method of selecting the gradation reproducing characteristic suitable to reproduce a line image by using the gradation control circuit 10.
FIG. 4 shows a gradation reproducing characteristic which is obtained after the gradation correction suitable for reproduction of a fine image was executed by using the gradation control circuit 10. As shown in FIG. 4, by performing a stepwise gradation reproduction, line images such as characters, fine lines, and the like of various densities in a range from a low density to a high density can be clearly reproduced at a uniform density.
However, in the case where the gradation reproduction as mentioned above is performed, there is a drawback such that when a fine image having a density near the density at which the gradation is switched is reproduced, the reproduction density of the fine image lies between two steps of the gradation reproducing characteristic and a stepwise density change considerably occurs in the fine image.
Consequently, when the copying operation is executed a plurality of times, such a density change becomes large so that it cannot be ignored.
Further, although there has hitherto been known a method of executing different processes to the line image and gradation image, the ability to realize uniform density of a line image when mixed with a gradation image has not yet been achieved. Further still, such an ability has not yet been achieved in the formation of a color image.